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Time Travel
Every day, we move through space in three dimensions: up and down, forward and backward, left and right. But we can move through time in only one direction, from the present to the future. We can only move forward in time. Despite this difference, Einstein believed that space and time were closely related, and could be described in one equation as space-time. To explain how things move, we must see time as the fourth dimension. One implication of Einstein's theories is that it is possible to travel through space-time—or into the past or future. Some scientists today believe that according to the laws of physics, time travel is possible. There are a few different theories about how a time machine could work. One is that it would have to exceed the speed of light. Another theory suggests that time travel may be possible by traveling into a black hole. Scientists also believe time travel is possible through a wormhole, a tunnel into another region of space-time.

Atomic Clocks
Atomic clocks are the most accurate clocks available. Atomic clocks are essential for the workings of GPS (Global Positioning System) navigation, the Internet, even the launching of space probes. Like a standard clock, atomic clocks rely on oscillations or vibrations to track time. But in an atomic clock, these oscillations occur between the nucleus of an atom and the surrounding electrons. The first atomic clocks used the vibrations from ammonia molecules. Since the 1950s, the most accurate atomic clocks have used cesium atoms as the vibrating source. An atomic clock can be so accurate, it would lose or gain no more than one second every three million years.